The spectroscopy of DABCO·Krn (1,4-diazabicyclo[2.2.2]octane) clusters was investigated using a two-color mass-selected ionization process. The transition from the ground state to the first excited state of DABCO (which is already a Rydberg state) is one photon forbidden and was the object of our studies. In the mixed cluster, this transition becomes allowed due to the reduced symmetry and its origin was detected in DABCO·Krn clusters for n = 1-3. Structural isomers of these clusters were identified and characterized by the spectral shifts of both the S0 → S1 and the S0 → IP transitions using combination rules. The blue shifts which were observed for the S1 Rydberg state differ from previous reports of a red shift for a valence state undergoing the clustering process.